1. Field of the Invention
The present invention relates to an image processing technique, and particularly to color deviation correction in a sequential transfer of images of multiple colors formed by developing units provided for the respective multiple colors.
2. Description of the Related Art
To achieve high speed image formation, many recent electrophotographic color image formation apparatuses include the same number of developing devices and photoconductors as the number of color materials and employ a system in which images of different colors are sequentially transferred to an image transport belt and then to a recording medium.
Using such a system (i.e., a tandem system) can drastically reduce throughput time, but on the other hand generates problems attributable to factors such as nonuniformity and poor attachment position accuracy of lenses of a deflection scanner and poor attachment position accuracy of the deflection scanner itself to a main body of the image formation apparatus. More specifically, when a scan line inclines or curves and when each color has a different degree of the inclination or curvature, a color deviation occurs due to a positional deviation of each of the colors on a transfer sheet. This problem consequently makes it difficult to obtain a high-quality color image.
As a countermeasure against such a color deviation, Japanese Patent Laid-Open No. 2002-116394 (referred to as Patent Document 1 below), for example, describes a method in which the magnitude of a curvature of a scan line is measured using an optical sensor in an assembly step of a deflection scanner, and the deflection scanner is fixed after the curvature of the scan line is adjusted by mechanically rotating a lens.
Japanese Patent Laid-Open No. 2003-241131 (referred to as Patent Document 2 below) describes a method in which the magnitude of an inclination of a scan line is measured using an optical sensor in a step of attaching a deflection scanner to a main body of an image formation apparatus, and the deflection scanner is attached to the main body of the apparatus while mechanically inclining the deflection scanner to adjust the inclination of the scan line.
To correct a light path of an optical system, a correction optical system including a light source and an f-θ lens, a mirror on the light path, or the like needs to be mechanically moved to make the registration of a test toner image. For this reason, the methods described in Patent Documents 1 and 2 require a highly-accurate, movable member, which entails an increase in costs.
Further, since it takes time to complete the correction of the light path of an optical system, it is impossible to carry out the correction frequently. However, the deviation of the light path changes under the influence of a factor such as an increase in the machine temperature. Even if the correction is performed at a certain point, the influence of the increase in the machine temperature cannot be eliminated. Accordingly, it is difficult to prevent a color deviation by correcting the light path of the optical system.
Meanwhile, Japanese Patent Laid-Open No. 2004-170755 (referred to as Patent Document 3 below) describes a method in which an inclination and a curvature of a scan line is measured using an optical sensor, bitmap image data is corrected so that the inclination and the curvature may be canceled, and an image is formed based on the data thus corrected. This method employs electrical correction which involves processing of image data, and therefore requires neither a mechanical adjustment member nor an adjustment step at the time of assembly. In this respect, the method according to Patent Document 3 can deal with a color deviation less expensively than those according to Patent Documents 1 and 2.
Such electrical color deviation correction is divided into one-pixel-basis correction and less-than-one-pixel-basis correction. The one-pixel-basis correction offsets a pixel in a sub scanning direction on a pixel unit according to a correction amount of an inclination and a curvature. The less-than-one-pixel-basis correction, on the other hand, corrects the gradation value of preceding and succeeding pixels in the sub scanning direction. This less-than-one-pixel-basis correction can achieve cancellation of an awkward step generated at a border where a pixel is offset by the one-pixel-basis correction, and thus can achieve smoothing out of the image.
One of negative effects of the electrical color deviation correction is unevenness in density in a minute image due to the less-than-one-pixel-basis correction.
FIG. 1 is a diagram illustrating such unevenness in density in a minute image. An input image 101 in FIG. 1 is a fine line having a constant gradation value. An image 102 is formed by actually performing color deviation correction on the input image 101. Even though the input image 101 is an image with a constant image gradation value, the output image formed based on the image 102 after the color deviation correction is an image of a fine line with uneven density. This is because an electrophotographic image formation apparatus is generally poor at forming an isolated image while keeping the proportional relation between the image gradation value and the actual image density value. This influence appears noticeably as unevenness in density in a minute image formed of fine lines. Reference numeral 103 shows how toner density is uneven when the image 102 after the color deviation correction is actually formed.
One of countermeasures against the unevenness in density in a minute image is to avoid performing the less-than-one-pixel-basis correction on the minute image. Specifically, an image is binarized, and the binarized image is then compared with a smoothing determination pattern stored in advance. When the image matches the pattern, the less-than-one-pixel-basis correction is not performed, and when the image does not match the pattern, the less-than-one-pixel-basis correction is performed.
However, when the less-than-one-pixel-basis correction is not performed, an awkward step is generated at a border where a pixel is offset by the one-pixel-basis correction, as described above. Such a step generated by the offset is noticeable particularly in a text with a small point size, which is a minute image.